An important component of disposable absorbent articles such as diapers is an absorbent core structure comprising water-absorbing polymers, e.g. hydrogel-forming and/or water-swellable polymers, also referred to as absorbent gelling material, AGM, or super-absorbent polymers, or SAP's. This polymer material ensures that large amounts of bodily fluids, e.g. urine, can be absorbed by the article during its use and locked away, thus providing low rewet and good skin dryness.
Especially useful water-absorbing polymers or SAP's are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like in the presence of cross-linking compounds, such as (relatively small amounts of) di- or poly-functional monomers such as N,N′-methylenebisacrylamide, trimethylolpropane triacrylate, ethylene glycol di(meth)acrylate, or triallylamine. The di- or poly-functional monomer materials serve to cross-link the polymer chains thereby rendering them water-insoluble, yet water-swellable. These cross-linked absorbent polymers contain a multiplicity of carboxylate groups attached to the polymer backbone. It is generally believed, that the neutralized carboxylate groups generate an osmotic driving force for the absorption of body fluids by the cross-linked polymer network.
Such reactions are typically done with aqueous solutions of the monomers and crosslinking agents, because a solution polymerization has been found to be most efficient and effective to provide (uniformity in) polymers, and water is typically the preferred reaction solvent (e.g. for safety and cost reasons). Thus, typically electrolyte monomers, such as (partially) neutralized unsaturated carboxylic acids (i.e. unsaturated carboxylate salts), and crosslinking agents are used in the form of an aqueous solution (the resulting water-absorbing polymers being cross-linked polyelectrolyte (e.g. polycarboxylate) polymers).
In recent years, some absorbent polymers that are linked by nano-sized clay particles have been proposed. Unlike some superabsorbent material whereby clay is added after polymerization, it has been found to be important that the clay is added in nano-size prior to polymerization, to ensure the clay form strong links between the polymers. This is for example described in “Nanocomposite Polymer Gels”; Schexnailder/Schmidt; Coloid Polym Sci (2009) 287: 1-11. Some of said clay linked polymers form elastic or stretchable hydrogels upon swelling. For example, water-containing hydrogel shaped or molded articles, comprising certain specific isopropyl polyamides linked by certain clay particles are described in Macromolecules 2002, 35, 10162-10171 (Kazutoshi Haraguchi et all); these elastic, shaped hydrogels are intended for medical purposes where they can be used in applications where they can de-water quickly, and thus shrink, upon demand, e.g. driven by temperature changes. WO 2009/041870 and WO 2009/041903 describe the desire to make clay linked polyacrylates, but that polyacrylates cannot be linked by nano-size clay particles successfully, because the clay aggregates in the presence of acrylate or acrylic acid. They teach thereto fibers, foams and films (that may be made in particles) of clay linked hydrogels, made by mixing nano-size clay particles and acrylic esters in a liquid to form clay linked polyacrylic esters, that may be shaped in foams, fibers, films etc. These polyacrylic ester shapes are then hydrolyzed using conventional hydrolysis techniques in order to obtain polyacrylate shapes. However, the hydrolyses of complete foams, fibers or gels, or even batches of finished particles of polyacrylic esters is a very slow and energy-demanding process, because the penetration of the hydrolysis solution is driven by diffusion only which is a generally slow process. Furthermore, it is difficult to achieve a very homogeneous hydrolysis throughout the entire polymer shapes, even if such shapes are made into particles, i.e. some parts of the polymer may be hydrolyzed earlier and to a larger extent than others. Furthermore, by-products from the hydrolysis (such as methanol or ethanol) would need to be removed from the product, and the level of these by-products would need to be brought to very low levels (toxicity, odour).
Thus, the proposed clay linked polyester shapes, and the hydrolysis thereof are not suitable for commercial scale production of (e.g. particulate) clay linked polyacrylates (particles).
The present invention however provides polymerization reaction liquids that allow homogeneous clay platelet dispersion in the presence of the polymerizable carboxylate/carboxylic acid monomers or oligomers, i.e. in an acid liquid, without or with very little aggregation of clay. The dispersed clay platelets can thus link the polymers during polymerization in a very uniform manner. Furthermore, the process is such that no subsequent ester-hydrolysis is required. The thus obtained clay linked polycarboxylate/polycarboxylic acid polymers can be suitably incorporated in absorbent articles.